Prosthetic heart valves are used to replace damaged or diseased heart valves, including the aortic, mitral (bicuspid), tricuspid, and pulmonary heart valves. There are two basic types of prosthetic heart valves, mechanical and tissue-type valves. Mechanical heart valves use a pivoting mechanical closure to provide unidirectional blood flow, while tissue type valves are made from natural tissue valve leaflets. Mechanical valves are made of pyrolytic carbon, and although they do not wear out, the patient in which the valve is implanted requires life-long anticoagulation, which is associated with an increased incidence of thrombotic and hemorrhagic complications. Tissue valves resemble native valves, and do not require life-long anticoagulation, but they wear out over time (in general after about 10 years). In fact, the tissue they are made of is a simple, flat piece of pericardium, and does not possess the structural characteristics necessary for long-term durability. Other types of tissue valves utilize animal valves, but the natural (dead) tissues degrade and calcify over time. Thus, there is a continued need for heart valves with improved functional properties.